1. Field of the Invention
This invention relates generally to antenna system design and more particularly to radar systems that utilize sidelobe blanking (SLB) for suppressing interference signals.
2. Description of Related Art
In many radar applications, stray signals may enter the radar through the sidelobes of the main antenna and may be interpreted as main beam signals. This results in false detections and angle error. Sidelobe blanking (SLB) has been commonly used to suppress such signals when they are impulsive (low duty cycle) whether they are due to sidelobe jammers, strong targets or discrete clutter echoes. The principle of SLB is to inhibit detection when such signals appear in the radar sidelobes.
In his 1968 paper entitled “Performance of Sidelobe Blanking Systems”, Maisel (Pub. Info) introduced what has come to be regarded as the classic SLB architecture. Maisel was concerned with the case where detection was based on a single radar pulse from a target with a constant radar cross section (RCS) and defined expressions for some of the relevant performance probabilities for evaluating SLB characteristics. These include Probabilities of Blanking (Pb), Probabilities of False Alarm (Pfa) and Probabilities of Detection (Pd). Farina and Shnidman have extended Marisel's work to include some expressions for certain types of target fluctuations.
The classic SLB architecture defined by Maisel has two antennas and associated receiver channels (i.e. main and auxiliary channels). The main antenna exhibits a relatively narrow main beam to interrogate its field of view and receive corresponding echo radiation therefrom. The auxiliary antenna, implemented as a single omni-directional element to the antenna system, exhibits a much broader main beam having less gain compared to the narrow main beam to receive the echo radiation. The auxiliary antenna gain is designed to be higher than the maximum sidelobe level of the main channel antenna pattern. The SLB logic circuits compare the signals processed by the two channels and determine whether or not to blank the main radar channel (i.e. decides that the echoes come from the sidelobes and blanks the echo). When the main channel output is larger than a suitable blanking threshold level, that is based on the auxiliary channel output, the main channel signal is processed as usual (i.e. it is submitted to the conventional circuits of the radar to ascertain whether a target is present in the searched direction). If this is not the case, then the main channel signal is inhibited or blanked (i.e. decided that the echoes come from the sidelobes and the echo is blanked).
As illustrated in FIG. 4A, the main goal of the SLB design is to select the parameter gain margin β, which is the difference (in dB) between the sidelobes δ of the main antenna and the auxiliary antenna gain ω. The numerical calculations of the Pb, Pfa and Pd parameters are necessary to select the proper values of F, ω, β and consequently of the auxiliary antenna gain ω, given the maximum sidelobe level Gω. These calculations are described in the article entitled “Design of SLB systems in the presence of correlated ground clutter” by A. Farina and F. Gini published in IEE Proceedings—Radar, Sonar Navigation, Vol. 147 No. August 2000. The use of Pb, Pd and Pfa rely on many system parameters including SLB gain margin, detection threshold, SLB threshold, target signal to noise ratio, interference to noise ratio (including jammer and clutter) and SLB configuration (before and after detection). One problem with this approach is that it is difficult to make comparison with multiple variables. Thus, the results do not reflect the performance of SLB antennas but instead are controlled by the system parameters. Further, the number of described approaches that compare the probabilities of blanking illustrate that there are too many parameters to manage.
Another approach involves comparing the cumulative percentage of gain margin. In this case as illustrated in FIG. 3, the method involves comparing the cumulative percentage of gain margin defined as the gain difference between the auxiliary and main antennas at corresponding angular positions as denoted by the shaded area in FIG. 3. The diagram also illustrates a typical distribution of the gain margin in a bell shaped curve. As shown, the cumulative percentage is usually reversed (one minus the percentage). The disadvantage is that this method relies on a single point comparison which is inaccurate.
The above prior art approach is further illustrated as follows. FIGS. 4B, 4C and 4D illustrate an example of a set of antenna patterns corresponding to a main beam pattern and auxiliary A and B sidelobe patterns. FIG. 4E illustrates a v-plane cut of the antenna patterns, the auxiliary B pattern appears low because the v-plane cut is on its null. FIG. 4F illustrates cumulative percentage at 0 gain margin. That is, the results of the second approach of computing cumulative percentage of the gain margin of the auxiliary A and auxiliary B sidelobe patterns. Comparing the cumulative percentage of gain shows the auxiliary A sidelobe pattern having a higher cumulative percentage at gain margin values>0 while the auxiliary B sidelobe pattern has higher cumulative percentage at gain margins<0. Thus, from this, utilizing the single gain point of 0, it is difficult to tell which antenna will perform better.
In addition to the above, the SLB systems discussed above are usually fixed during their design and hence they are not adaptive during real time operations. That is, SLB is effective at identifying interference from pulsed interference sources in the sidelobe directions. Thus, when there are platform motion or element or sub-array failures, SLB becomes less effective at identifying interference.
Accordingly, it is an object to provide a method of providing an effective SLB capability for inclusion in an antenna design.
It is another object of the present invention to provide a method of providing effective SLB during real time operation.